Teach libvcc how to set STRING += STRING;

Refs #3100

bin/varnishtest/tests/v00018.vtc

@@ -27,11 +27,6 @@ varnish v1 -errvcl {Expected '=' got '+='} {
 	sub vcl_backend_fetch { set bereq.backend += b; }
 }
 
-varnish v1 -errvcl {Expected '=' got '+='} {
-	backend b { .host = "127.0.0.1"; }
-	sub vcl_recv { set req.url += now; }
-}
-
 varnish v1 -errvcl {Expected ';' got 'if'} {
 	backend b { .host = "127.0.0.1"; }
 	/* XXX: This should not really be an synth */

lib/libvcc/vcc_action.c

@@ -80,6 +80,7 @@ static const struct arith {
 	{ DURATION,	T_DIV,		REAL, "\v / " },
 	{ DURATION,	'=',		DURATION },
 	{ DURATION,	0,		DURATION },
+	{ STRING,	T_INCR,		STRING, "\v,\n" },
 	{ VOID,		'=',		VOID }
 };